kinda depends on how much your planning to spend, a morebeer.com gift certificate would be the easiest

but a nice stainless steel mash paddle never hurts
maybe a few "better bottle" carboys

and if your really stumped morebeer.com/search/103521 more beer's holiday gift guide


(yes, I like morebeer, they're local to me and have treated me well for a long time)

No plastic vessels. Only glass & Stainless Steel.

Any way it's such a specialist hobby with so many levels of technical acumen and the hard-ware that goes with each level that, in order to know for sure what he'd like, you'd need to be as current in the art as he is or have heard him express an interest in something specific.
Even in the latter instance it's easy to screw up unless you have heard him articulate real detailed specifics.

Does he top ferment or bottom?
Is he into steam beers or ales or lagers?
Does he toast his malt, buy it dried, or canned as a syrup?
Does he inject CO2 or ferment in the bottle?
questions questions questions
and each answer would lead to more questions lots more.

i got a kegging system for the holidays! woohoo. a little on the pricey side, but i would never have gotten it myself, so that might be nice. on the less expensive side, more carboys are always a plus, more kegs if he already has the regulator/tank/tap, but some quality personalized glassware (don't forget a pitcher) would satisfy most homebrewers i think. also, rumor has it new hops shipments are in the next few weeks, so any of the noble hops would be a welcome gift for many a brewer (hallertau and tettnang are on my list)